This invention relates to a method for injection molding of a disk shaped substrate for an optical information record carrier formed of polycarbonate resin, such as the magneto-optical recording medium or the write once type recording medium.
The optical recording system can record and/or reproduce in contact-free manner, can be handled easily and is invulnerable to demage or contamination, while it has the recording capacity about tens or hundreds of times larger than that of the conventional magnetic recording system. Thus it is practically used in a compact disk having digitally recorded audio signals or a video disk having recorded video signals, while it is expected to be used in large capacity information record file, such as code or image information.
Various types of the optical recording media of the optical recording system are known in addition to the aforementioned compact disks or video disks, such as write once type optical disk, erasable optical disk or the magneto-optical disk. These disks are comprised of a recording layer for the optical information formed on an transparent disk of polycarbonate resin or PMMA (polymethethylmethacrylate) resin. Various demands have been raised in forming the disks.
Typical of these demands are the reduced double refraction which means phases shift between the disk signal readout incident light and reflected light and, mainly is caused by the internal strain of the disk, optimum transfer properties and surface smoothness, reduced contamination and reduced deflection (skew) of the formed surface. Above all, it is required of the magneto-optical disk to reduce double refraction caused in the transparent disk, since it is adapted to read out as signals only minute rotations of the plane of polarization of the irradiated laser light.
Under these circumstances, evolution in the art of injection molding is carried out in many aspects so that it is now possible to produce the polycarbonate resin disk which is almost completely free of double refraction at the time of forming.
However, as a result of our further investigations, it has been found that, while it is possible to produce the disk showing only reduced double refraction at the initial stage after termination of the molding under the conventional injection molding art, the double refraction when being used was changed chronologically and, above all, shifted towards the minus side, thus resulting in increase of the double refraction. The tendency is outstanding in an internal area with a magneto-optical disk having a central hub for assuring the chucking accuracy relative to the recording and reproducing apparatus. For example, when it is used in the apparatus, it is becoming difficult to assure a normal operation because of increased double refraction.
In the present invention, the polarity of double refraction that is, minus or plus, is defined in such a manner that the direction in which double refraction on the inside area of the formed disk heated to 100.degree. C. is changed is defined as minus, with the direction opposite thereto being defined as plus. The double refraction is a phenomenon that takes place when the light passes through a medium having variable refractive indices along the directions within the same plane, such medium being anisotropic with respect to the refractive index. For example, with the refractive index in a prescribed direction (x-direction) as n.sub.x and with that in a direction orthogonal thereto (y-direction) as n.sub.y, phase shift is caused between the x-component (the light having the plane of polarization parallel to the x-direction) and the y-component (the light having the plane of polarization parallel to the y-direction). The phase difference .delta. is given by EQU .delta.=2.lambda./.pi.. (n.sub.x -n.sub.y)d . . . (1)
wherein .delta.: wavelength of light and d: the distance in the medium traversed by light. The direction in which the phase difference .delta. of the transmitted light is changed chronologically or under the aforementioned heating conditions was defined as the minus direction, with the direction opposite thereto being defined as the plus direction.
Although it may be thought to produce the disk having double refraction previously set to the plus side, in order to manage the aforementioned chronological changes, the current situation is necessarily to sacrifice other properties.
That is, in order to obtain the plus double refraction of the produced disk, it is necessary to increase the temperature of the resin or the mold during the injection molding. However an increase in the mold temperature results in curved or warped disk and loss of smoothness, while an increase in the resin temperature results in decomposition of the resin and increased contamination. When forming under a lesser pressure, it is possible to produce a disk having a plus double refraction. In such case, however, transfer characteristics are lowered, while it becomes difficult to assure a flatness of the hub attaching reference plane and skew is also increased.